Production of paper, board and cardboard

ABSTRACT

Paper, board and cardboard are produced by shearing the paper stock, adding a microparticle system comprising a cationic polymer and a finely divided inorganic component to the paper stock after the last shearing stage before the head box, draining the paper stock with sheet formation and drying the sheets, by a process in which cationic polyacrylamides, polymers containing vinylamine units and/or polydiallyldimethylammonium chloride having an average molar mass Mw of in each case at least 500 000 Dalton and a charge density of in each case not more than 4.0 meq/g are used as cationic polymers of the microparticle system.

The present invention relates to a process for the production of paper,board and cardboard by shearing the paper stock, adding a microparticlesystem comprising a cationic polymer and a finely divided inorganiccomponent to the paper stock after the last shearing stage before thehead box, draining the paper stock with sheet formation and drying thesheets.

The use of combinations of nonionic or anionic polymers and bentonite asretention aids in the production of paper is disclosed, for example, inU.S. Pat. No. 3,052,595 and EP-A-0 017 353.

EP-A-0 223 223 discloses a process for the production of paper and boardby draining a-paper stock, first bentonite being added to a paper stockhaving a consistency of from 2.5 to 5% by weight, the paper stock thenbeing diluted, a highly cationic polymer having a charge density of atleast 4 meq/g being added and finally a high molecular weight polymerbased on acrylamide being added and the pulp thus obtained being drainedafter thorough mixing.

According to the process disclosed in EP-A-0 235 893 for the productionof paper, first a substantially linear synthetic cationic polymer havinga molar mass of more than 500 000 is first metered to an aqueous fibersuspension in an amount of more than 0.03% by weight, based on dry paperstock, the mixture is then subjected to the action of a shear field, theinitially formed flocks being divided into microflocks which carry acationic charge, bentonite then being metered and the pulp thus obtainedbeing drained without further action of shear forces.

EP-A-0 335 575 describes a papermaking process in which two differentwater-soluble, cationic polymers are added in succession to the pulp,and the pulp is then subjected to at least one shearing stage and isthen flocculated by addition of bentonite.

EP-A-0 885 328 describes a process for the production of paper, acationic polymer first being metered into an aqueous fiber suspension,the mixture then being subjected to the action of a shear field, anactivated bentonite dispersion then being added and the pulp thusobtained being drained.

EP-A 0 711 371 discloses a further process for the production of paper.In this process, a synthetic, cationic, high molecular weight polymer isadded to a thick stock cellulosic suspension. After dilution of theflocculated thick stock and before drainage, a coagulant which consistsof an inorganic coagulant and/or a second, low molecular weight andhighly cationic water-soluble polymer is added.

EP-A-0 910 701 describes a process for the production of paper andcardboard, a low molecular weight or medium molecular weight cationicpolymer based on polyethylenimine or polyvinylamine and then a highmolecular weight cationic polymer, such as polyacrylamide,polyvinylamine or cationic starch, being added in succession to thepaper pulp. After this pulp has been subjected to at least one shearingstage, it is flocculated by adding bentonite and the paper stock isdrained.

EP-A-0 608 986 discloses the metering of a cationic retention aid intothe thick stock in papermaking. A further process for the production ofpaper and cardboard is disclosed in U.S. Pat. No. 5,393,381,WO-A-99/66130 and WO-A-99/63159, a microparticle system comprising acationic polymer and bentonite likewise being used. The cationic polymerused is a water-soluble, branched polyacrylamide.

WO-A-01/34910 describes a process for the production of paper, in whicha polysaccharide or a synthetic, high molecular weight polymer ismetered into the paper stock suspension. Mechanical shearing of thepaper stock must then be carried out. The reflocculation is effected bymetering an inorganic component, such as silica, bentonite or clay, anda water-soluble polymer.

U.S. Pat. No. 6,103,065 discloses a process for improving the retentionand the draining of paper stocks, a cationic polymer having a molar massof from 100 000 to 2 million and a charge density of more than 4.0 meq/gbeing added to the paper stock after the final shearing, a polymerhaving a molar mass of at least 2 million and a charge density of lessthan 4.0 meq/g being added simultaneously or thereafter and bentonitethen being metered. In this process, it is not necessary to subject thepaper stock to shearing after the addition of the polymer. After theaddition of the polymer and of the bentonite, the pulp can be drainedwith sheet formation without the further action of shear forces.

In the known papermaking processes, in which a microparticle system isused as a retention aid, relatively large amounts of polymer andbentonite are required. Those processes in which the presence ofcationic polymers having a charge density of more than 4.0 is absolutelyessential give papers which tend to yellow.

It is an object of the present invention to provide a further processfor the production of paper with the use of a microparticle system,smaller amounts of polymer and bentonite being required in comparisonwith the known processes and at the same time improved retention anddrainage being achieved and papers which have less tendency to yellowingbeing obtained.

We have found that this object is achieved, according to the invention,by a process for the production of paper, board and cardboard byshearing the paper stock, adding a microparticle system comprising acationic polymer and a finely divided inorganic component to the paperstock after the last shearing stage before the head box, draining thepaper stock with sheet formation and drying the sheets, if cationicpolyacrylamides, polymers containing vinylamine units and/orpolydiallyldimethylammonium chloride having an average molar mass Mw ofin each case at least 500 000 Dalton and a charge density of in eachcase not more than 4.0 meq/g are used as cationic polymers of themicroparticle system, the microparticle system used as a retention aidbeing free of polymers having a charge density of more than 4 meq/g.

All paper grades, for example cardboard, single-layer/multilayer foldingboxboard, single-layer/multilayer liner, fluting medium, papers fornewsprint, medium writing and printing papers, natural gravure papersand light-weight coating papers, can be produced by the novel process.To produce such papers, it is possible to start, for example, fromgroundwood, thermomechanical pulp (TMP), chemothermomechanical pulp(CTMP), pressure groundwood (PGW), mechanical pulp and sulfite andsulfate pulp. The pulps may be both short-fiber and long-fiber.Wood-free grades which give very white paper products are preferablyproduced by the novel process.

The papers can, if required, contain up to 40, in general from 5 to 35,% by weight of fillers. Suitable fillers are, for example, titaniumdioxide, natural and precipitated chalk, talc, kaolin, satin white,calcium sulfate, barium sulfate, clay and alumina.

According to the invention, the microparticle system consists of acationic polymer and a finely divided anionic component. Suitablecationic polymers are cationic polyacrylamides, polymers containingvinylamine units, polydiallyldimethylammonium chlorides or mixturesthereof, having an average molar mass Mw of, in each case, at least 500000 Dalton and a charge density of, in each case, not more than 4.0meq/g. Cationic polyacrylamides having an average molar mass Mw of atleast 5 million Dalton and a charge density of from 0.1 to 3.5 meq/g andpolyvinylamines which are obtainable by hydrolysis of polymerscontaining vinylformamide units are particularly preferred, the degreeof hydrolysis of the vinylformamide units being from 20 to 100 mol % andthe average molar mass of the polyvinylamines being at least 2 000 000Dalton. The polyvinylamines are preferably prepared by hydrolysis ofhomopolymers of vinylformamide, the degree of hydrolysis being, forexample, from 70 to 95%.

Cationic polyacrylamides-are, for example, copolymers which areobtainable by copolymerization of acrylamide and at least one di-C1- toC2-alkylamino-C2- to C4-alkyl (meth)acrylate or a basic acrylamide inthe form of the free bases, of the salts with organic or inorganic acidsor of the compounds quaternized with alkyl halides. Examples of suchcompounds are dimethylaminoethyl methacrylate, diethylaminoethylmethacrylate, dimethylaminoethyl acrylate, diethylaminoethyl acrylate,dimethylaminopropyl methacrylate, dimethylaminopropyl acrylate,diethylaminopropyl methacrylate, diethylaminopropyl acrylate and/ordimethylaminoethylacrylamide. Further examples of cationicpolyacrylamides and polymers containing vinylamine units are describedin the publications mentioned in connection with the prior art, such asEP-A-0 910 701 and U.S. Pat. No. 6,103,065. Both linear and branchedpolyacrylamides may be used. Such polymers are commercial products.Branched polymers, which can be prepared, for example, bycopolymerization of acrylamide or methacrylamide with at least onecationic monomer in the presence of small amounts of crosslinkingagents, are described, for example, in the publications U.S. Pat. No.5,393,381, WO-A-99/66130 and WO-A-99/63159 mentioned in connection withthe prior art.

Further suitable cationic polymers are polydiallyldimethylammoniumchlorides (polyDADMAC) having an average molar mass of at least 500 000,preferably at least 1 million, Dalton. Polymers of this type arecommercial products.

The cationic polymers of the microparticle system are added to the paperstock in an amount of from 0.005 to 0.5, preferably from 0.01 to 0.2, %by weight.

Suitable inorganic components of the microparticle system are, forexample, bentonite, colloidal silica, silicates and/or calciumcarbonate. Colloidal silica is to be understood as meaning productswhich are based on silicates, e.g. silica microgel, silica sol,polysilicates, aluminum silicates, borosilicates, polyborosilicates,clay or zeolites. Calcium carbonate can be used, for example, in theform of chalk, milled calcium carbonate or precipitated calciumcarbonate as the inorganic component of the microparticle system.Bentonite is generally understood as meaning sheet silicates which areswellable in water. These are in particular the clay mineralmontmorillonite and similar clay minerals, such as nontronite,hectorite, saponite, sauconite, beidellite, allervardite, illite,halloysite, attapulgite and sepiolite. These sheet silicates arepreferably activated prior to their use, i.e. converted into a formswellable in water, by treating the sheet silicates with an aqueousbase, such as aqueous solutions of sodium hydroxide, potassiumhydroxide, sodium carbonate or potassium carbonate. A preferably usedinorganic component of the microparticle system is bentonite in the formtreated with sodium hydroxide solution. The platelet diameter of thebentonite dispersed in water, in the form treated with sodium hydroxidesolution, is for example from 1 to 2 μm and the thickness of theplatelets is about 1 nm. Depending on type and activation, the bentonitehas a specific surface area of from 60 to 800 m²/g. Typical bentonitesare described, for example, in EP-B-0235893. In the papermaking process,bentonite is added to the cellulose suspension typically in the form ofan aqueous bentonite slurry. This bentonite slurry may contain up to 10%by weight of bentonite. Usually, the slurries contain about 3-5% byweight of bentonite.

The colloidal silica used may be a product from the group consisting ofsilicon-based particles, silica microgels, silica sols, aluminumsilicates, borosilicates, polyborosilicates and zeolites. These have aspecific surface area of 50-1 000 m²/g and an average particle sizedistribution of 1-250 nm, usually 40-100 nm. The preparation of suchcomponents is described, for example, in EP-A-0041056, EP-A-0185068 andU.S. Pat. No. 5,176,891.

Clay or kaolin is a water-containing aluminum silicate having a lamellarstructure. The crystals have a layer structure and an aspect ratio(ratio of diameter to thickness) of up to 30:1. The particle size issuch that at least 50% of the particles are smaller than 2 μm.

Carbonates used, preferably calcium carbonate, may be ground calciumcarbonate (GCC) or precipitated calcium carbonate (PCC). GCC is preparedby milling and classification processes with the use of millingassistants. It has a particle size such that 40-95% of the particles aresmaller than 2 μm, and the specific surface area is 6-13 m²/g. PCC isprepared by passing carbon dioxide into calcium hydroxide solution. Theaverage particle size is 0.03-0.6 μm and the specific surface area canbe greatly influenced by the choice of the precipitation conditions. Itis 6-13 m²/g.

The inorganic component of the microparticle system is added to thepaper stock in an amount of from 0.01 to 1.0, preferably from 0.1 to0.5, % by weight.

The consistency of the pulp is, for example, from 1 to 100, preferablyfrom 4 to 30, g/l. The aqueous fiber suspension is subjected to at leastone shearing stage. It passes through at least one cleaning, mixingand/or pumping stage. Shearing of the pulp can be effected, for example,in a pulper, screen or refiner. After the final shearing stage andbefore the head box, according to the invention, the microparticlesystem is metered onto the wire. A procedure in which first the cationicpolymer and then the inorganic component of the microparticle system ismetered into the paper stock, which has been subjected to shearingbeforehand, is particularly preferred here. However, it is also possibleto meter first the inorganic component of the microparticle system andthen the cationic polymer or to add both components simultaneously tothe paper stock. Draining of the paper stock is then carried out withoutfurther action of shear forces on a wire with sheet formation. The papersheets are then dried.

In addition to the microparticle system, the process chemicals usuallyused in papermaking can be added to the paper stock in the conventionalamounts, for example fixing agents, dry and wet strength agents, enginesizes, biocides and/or dyes.

Compared with the known processes, the novel process achieves anincrease in the retention of fines and fillers and of process chemicals,such as starch, dyes and wet strength agents, and an improvement in thedraining rate, without adversely affecting the formation and paperproperties. Moreover, a substantial improvement in the fiber recoveryand hence in the relief of the wastewater treatment plant is achieved.

In the examples, percentages are by weight, unless evident otherwisefrom the context.

The first pass retention (FP retention) was determined by calculatingthe ratio of the solids content in the white water to the solids contentin the head box. It is stated in percent.

The FPA retention (first pass ash retention) was determined analogouslyto the FP retention, but only the ash content was taken into account.

EXAMPLE 1

A paper stock comprising a wood-free, bleached pulp having a consistencyof 7 g/l and a filler content of 30% of calcium carbonate was processedon a Fourdrinier machine with a hybrid former to give a paper of writingand printing quality. The following arrangement of mixing and shearingmeans was used: mixing chest, dilution to 7 g/l, mixing pump, cleaner,head box pump, screen and head box. 32 t of paper were produced perhour.

After the screen (last shearing stage before the head box), first 270g/t of a commercial high molecular weight, cationic polyacrylamide.(Polymin PR 8140, average molar mass Mw 7 million) and 2 500 g/t ofbentonite were metered. The FP retention was 81.5% and the FPA retention60.2%.

Comparative Example 1

The example was repeated with the exceptions that 410 g/t of thecationic polyacrylamide were metered before the screen and the pump and3 000 g/t of bentonite after the screen and before the head box. Theseamounts were required in order to achieve a formation just as good as inthe example. The FP retention here was 79.9% and the FPA retention59.1%.

As shown by a comparison of the results of the example with the resultsof the comparative example, the saving of polymer was 30% and the savingof bentonite 17%. With equally good formation, it was possible in theexample according to the invention to achieve an improvement in theretention. The improvement in the drainage over the wire was about 10%.

EXAMPLE 2

A wood-containing paper stock comprising groundwood and chemical pulpand having a consistency of 7 g/l and a filler content of 30% of amixture of clay and calcium carbonate (1:1) was processed on a papermachine with a gap former to give a paper of LWC quality. The followingarrangement of mixing and shearing means was used: mixing chest,dilution, decolator, pump, screen, head box. 30 t of paper were producedper hour.

After the screen (final shearing stage before the head box), first 200g/t of a-commercial high molecular weight cationic polyacrylamide(Polymin KP 2520, average molar mass Mw 5 million) and 1 400 g/l ofbentonite were metered. The FP retention was 69% and the FPA retention40%.

Comparative Example 2

Example 2 was repeated with the exceptions that 280 g/t of the cationicpolyacrylamide were metered before the pump and the screen and 1 400 g/tof bentonite after the screen and before the head box. This amount wasrequired in order to achieve an equally good retention. The FP retentionhere was 69% and the FPA retention 40%.

As shown by a comparison of the results of example 2 with the results ofcomparative example 2, the saving of polymer was about 30%. Although asmaller amount of retention aid was used in example 2 than incomparative example 2, it was possible to achieve equally good formationand paper properties in example 2.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national stage application of International PatentApplication No. PCT/EP03/08037, filed on Jul. 23, 2003, and claimspriority to German Patent Application No. 102 36 252.1, filed on Aug. 7,2002, both of which are incorporated herein by reference in theirentireties.

1. A process for the production of paper, board or cardboard, saidprocess comprising: shearing a paper stock, adding a microparticlesystem comprising a cationic polymer and a finely divided inorganiccomponent to the paper stock after the last shearing stage before a headbox, draining the paper stock and forming a sheet, drying said sheet,wherein said cationic polymer is selected from the group consisting ofcationic polyacrylamide, a polymer comprising one or more vinylamineunits, polydiallyldimethylammonium chloride and mixtures thereof,wherein said cationic polymer has having an average molar mass Mw of atleast 500 000 Dalton and a charge density of not more than 4.0 meq/gand, the microparticle system is used as a retention aid and is free ofone or more polymers having a charge density of more than 4 meq/g.
 2. Aprocess as claimed in claim 1, wherein said cationic polymer is saidcationic polyacrylamide having an average molar mass Mw of at least 5million Dalton and a charge density of from 0.1 to 3.5 meq/g.
 3. Aprocess as claimed in claim 1, wherein said cationic polymer is saidpolymer comprising one or more vinylamine units obtained y hydrolysis ofa polymer comprising one or more vinylformamide units, the degree ofhydrolysis of the vinylformamide units being from 20 to 100 mol % andthe average molar mass of the polyvinylamines being at least 2 millionDalton.
 4. A process as claimed in claim 1, wherein the cationic polymerof the microparticle system is added to the paper stock in an amount offrom 0.005 to 0.5% by weight, based on dry paper stock.
 5. A process asclaimed in claim 1, wherein the cationic polymer of the microparticlesystem is added to the paper stock in an amount of from 0.01 to 0.2% byweight, based on dry paper stock.
 6. A process as claimed in claim 1,wherein said inorganic component is at least one material selected fromthe group consisting of bentonite, colloidal silica, silicate, calciumcarbonate, and mixtures thereof.
 7. A process as claimed in claim 1,wherein the inorganic component of the microparticle system is added tothe paper stock in an amount of from 0.01 to 1.0% by weight, based ondry paper stock.
 8. A process as claimed in claim 1, wherein theinorganic component of the microparticle system is added to the paperstock in an amount of from 0.1 to 0.5% by weight, based on dry paperstock.
 9. A process as claimed in claim 1, wherein the cationic polymeris metered into the paper stock and then the inorganic component of themicroparticle system is metered into the paper stock.